How to save and load a game?

I found a couple of old threads from 2005 about saving and loading files, but I'm still confused about how to do this.

Can someone possibly spell out this procedure to me? Here is a really simplified version of what I'm trying to do:

a) When the game loads, I want to grab "4" from a text file, savegame.txt.
b) I want to assign "4" to a global variable, $myValue.
c) When I close the game, I want to change "4" to "8" in the savegame.txt.

Any help would be much appreciated!